(a) Field of the Invention:
This invention relates to an endoscope illuminating optical system capable of a wide angle illumination.
(b) Description of the prior art:
The conventional endoscope optical system is formed of a light guide 2 embedded in an endoscope tip part 1 and a concave lens 3 secured to the front end surface of the tip part 1 so as to be positioned near the exit end of the light guide 2 as shown in FIG. 1. However, endoscopes having an observing optical system of a visual angle larger than 100 degrees have recently come to be used to widely observe a surface 4 of such object to be inspected as a body wall. There has been a problem that, with an illuminating optical system, the periphery of a visual field can not be brightly illuminated. Therefore, in order to improve this point, there is conceived such endoscope illuminating optical system as is mentioned, for example, in the Gazette of Japanese Patent Laid Open No. 20428/1981. This is formed of a single fiber 5 cemented to the exit end surface of the light guide 2 and a convex lens 6 secured to the front end surface of the tip part 1 so as to positioned in front of the single fiber as shown in FIG. 2. However, there has been a problem that, even if this illuminating optical system is used, in an endoscope of a visual angle larger than 120 degrees, the periphery of the visual field will be still dark. That is to say, as shown in FIG. 3, the principle of this illuminating optical system can be considered to be that the image of the exit end 5a of the single fiber 5 is projected on the surface 4 of the object to be inspected by the convex lens 6. The lights a, a' and a" illuminating the periphery of the visual field are the lights coming out of the edge of the exit end surface 5a and the lights illuminating the center P of the visual field are the lights coming out of a point Q in the single fiber 5. By the way, the point Q is not on the exit end surface 5a of the single fiber, because the convex lens 6 has a distortion and the exit end surface 5a projects forward. Further, the lights passing through the edge of the exit end surface 5a come also out of the zone A in the peripheral part of the exit end surface 2a of the light guide 2. As shown in FIG. 4, the zone A is of a shape enclosed with two arcs and the lights passing through the point Q come out of the circular zone B in the central part of the exit end surface 2a of the light guide 2. Therefore, as shown in FIG. 5, t;he light distribution characteristics in this case are such that the center of the visual field is bright and the peripheral part is gradually dark. With such light distribution characteristics, the surface 4 of the object to be inspected will be obstructed by the brightness in the center and will not be seen unless it is made brighter to some extent. That is to say, in fact, the part of the surface 4 of the object to be inspected on which the illuminating light little reaches will not be able to be observed. Particularly, in such case as of photographing, if the dark peripheral part is also to be photographed, the bright center part will be photographed to be really white (so bright as to be over-exposed). It is not practical. If the center is to be photographed so as to be definitely seen, the peripheral part will be so dark as to be photographed to be really black and the result will be the same as of a narrow illuminated range (narrow visual angle). By the way, the single fiber means a thick optical fiber provided with a core and clad and may be considered to be one made thick of optical fibers used for a light guide or the like.